Personal computers, computerised work stations and desktop publishing systems, for example, typically incorporate one or more electronic displays by which images are presented to the user to enable manipulation of such systems to perform certain functions and also for the mere presentation of information. Traditionally, such displays have comprised cathode ray tubes devices, which are more commonly known as video displays, and in which image data is displayed in a rasterised format either using interlaced sequential fields or in a non-interlaced manner. Such devices typically have an image display rate of 25 to 30 frames per second (ie. 50 to 60 fields per second), depending upon the particular standard being used. Such arrangements are also utilised for the display of television and other video type signals.
Images displayed on such devices typically comprise text, graphic object animation or symbols, and natural (photographic) type images. Because of the relatively high refresh rate of video displays, there is generally no need to distinguish between different image components in such systems in order to ensure the display of continuous, smooth images. Such arrangements typically require significant computational overhead to ensure regular maintenance of the display and the updating of the image portions thereon.
However, there exist display devices which can operate at lower refresh rates and which permit display management operation utilising lower computational overheads. An example of such a display is a ferroelectric liquid crystal field (FLCD) which can be operated at a refresh rate as low as six frames per second, thus permitting image latency well in excess of that detectable by the human eye. Such low refresh rates permit the computational overhead associated with display management to be reduced compared with that of cathode ray tube devices.
However, difficulties arise with utilising such low refresh for modern computing and desktop publishing applications, for example. In particular, where portions of the image comprise text, it is often the case that those portions of the image are only updated at a relatively slow rate that can be well accommodated by low refresh rates such as six frames per second. However, where complex images such as some animation and natural (photographic-type) images are to be displayed, low refresh rates can provide a significant deterioration in image quality. This is particularly the case with moving images when displayed on such devices, but can also apply to the display of relatively complex static images.
Some methods have been proposed to overcome this difficulty and include halftoning and dithering the image in different manners. However, whilst such methods can provide adequate results for a natural image for example, such systems are not so advantageous for text images, and vice versa. Arrangements have also been proposed which detect movement in portions of the display, utilising threshold techniques which permit identifying that portion of the display that needs to be refreshed at a higher rate. However, halftoning and dithering can give rise to noise in a displayed image that can be misconstrued as movement that can thus cause unnecessary updating of the display and accordingly an overuse of display management systems. These difficulties are also aggravated where the text is coloured on a coloured background, as opposed to the more traditional black on white or white on black text displays.
Accordingly, in order to ensure optimal use of die low refresh rate of such displays, a more efficient means of detecting and distinguishing between text and natural portions of an image is required.